The field of invention is coaxial cables having an inner conductor, a foam dielectric material formed about the inner conductor, and a shield formed about the dielectric material.
Coaxial cable is commonly used for many applications, such as transmission of radio frequency signals, cable television signals and cellular telephone broadcast signals. A coaxial cable of the type with which this invention concerns includes an inner conductor, a foam-type dielectric around the inner conductor, an electrically conductive shield surrounding the dielectric foam and serving as an outer conductor, and a protective jacket which surrounds the shield. The foam dielectric electrically insulates the inner conductor from the surrounding shield.
Commercially available coaxial cables which address the cost-sensitive mass market (exclusive of special purpose cable products) comprise basically four types: 1) braided shield cable; 2) smooth-walled cable; 3) annular corrugated cable; and 4) helical corrugated cable.
Braided shield cable is the lowest cost product and has excellent flexibility, however, it suffers badly in electrical properties. The braided shield has poor shielding effectiveness due to the porous woven nature of the shield, and typically requires the addition of a conductive foil under the braided shield to achieve even marginally acceptable shield effectiveness. Further, braided shield cable is ineffective in resisting intrusion of fluids, as the braid will actually xe2x80x9cwickxe2x80x9d fluids through the cable. The water blocking properties of braided shield cable can be improved by impregnating the braid with heavy grease, however this step raises the cost of the product. The braided shield is a loose braid that results in inconsistent contacts that creates non-linear joints. The effect of this is intermodulation, which is a type of noise or interference that is injected into the cable. Furthermore as noted, xe2x80x9cwaterproofingxe2x80x9d of braided cable requires the addition of a grease type material with the braid. However, this is a drawback in that it results in difficulty is attaching connectors to the cable, because the grease is emitted by the cable during attachment of the connector. Also, over time the cables are known to leak grease due to cracks or damage to the cable, and create an environmental problem.
xe2x80x9cSmooth-walledxe2x80x9d cable, as it is termed, typically comprises an aluminum tube as a shield and outer conductor. It is more costly than braided shield cable, however, because the shield is a solid tube, the shield effectiveness of this cable type is excellent. This product, however, has poor flexibility, requiring special tools to bend it, and suffers from intolerable kinking if the bends are not formed properly. Any such kinking dramatically impairs the electrical properties of the cable. Smooth-walled cable shields are welded using an HF (high frequency) welding process, as HF welding permits much faster line speeds than the TIG (tungsten inert gas) welding process universally used in the manufacture of helical and annular corrugated cable (to be described).
Near the high end of commercial coaxial cable is helical corrugated cable. Helical corrugated cable has a shield composed typically of copper. To form the shield, copper sheet, is wrapped around a foam dielectric core and welded. The welded copper tube is then corrugated using a corrugating die, which spins around the tube and imparts the corrugations as the tube is advanced. This xe2x80x9csingle leadxe2x80x9d corrugation process necessitates much slower line speeds than is possible with smooth-walled cable, but results in a much more flexible product than smooth-walled cable.
The use of copper as the shield material and the typically slow corrugation process drive up the cost of helical corrugated cable, however, its superior electrical and mechanical properties compensate in many applications for the increased cost. Helical corrugated cable suffers, however, by having less-than-optimum water blocking properties. Because the helical convolutions formed in the cable shield inherently create an uninterrupted passageway along the cable between the shield and the foam dielectric, water or other fluids entering the cable easily migrate along the cable. For this reason, helical corrugated cable is not recommended for use underground or in other aqueous environments.
At the high end of the four basic types of mass-marketed foam cable is annular corrugated copper cable. This product has all the attributes of helical corrugated copper cable, and in addition has improved water-blocking capability. Conventional copper annular corrugated cable with a foam dielectric, during its manufacture, has a tubular shield welded around foam dielectric with a space provided between the shield and the dielectric. The space is needed to permit the xe2x80x9cgatheringxe2x80x9d of the tubular material, as in the manufacture of conventional copper helical corrugated cable. This space commonly leads to the capturing of air within the annual corrugations formed. However, despite the air gaps thus formed, because the corrugations are annular, like 360-degree rings, which contact the dielectric foam, each ring acts as a sort of seal, resists water migration. The superior water blocking ability of annular corrugated cable, relative to helical corrugated cable, permits it to be used underground and in more demanding aqueous environments than helical corrugated cable. Further, for a given depth of corrugation, annular corrugated cable is somewhat more flexible than helical corrugated cable.
However, there is a price to be paid for the improved water blocking and flexibility of annular corrugated cable compared with helical corrugated cable. The process of forming annular corrugations is much slower than the process of manufacturing helical corrugations. The resulting slower line speeds add significant manufacturing cost. For example, typical industry line speeds for corrugating annular shield cable may be 50 percent slower than industry line speeds for corrugating helical shield cable. Furthermore, the annular corrugating process does not lend itself to producing high pitch-to-depth ratio cable. Accordingly, annular corrugated cable tends to be less flexible than helical corrugated cable.
Until the present invention, we know of no product which meets all four of the desired foam coaxial cable attributes: 1) low cost; 2) electrical properties including shield effectiveness and intermodulation suppression comparable to that of solid tubular shielded cable; 3) mechanical properties, primarily flexibility, comparable to corrugated cable; and 4) water blockage comparable to annular corrugated cable.
Trilogy Communications, Inc. manufactures a coaxial cable for indoor use only that has an air dielectric design. The cable has an aluminum outer conductor and a copper clad aluminum inner conductor. However, because air is used as the dielectric, periodic spacers being used to separate the inner and outer conductors, these cables are highly susceptible to fluid migration and therefore cannot be used outdoors, or in any wet environment. Further, air-dielectric cable is more expensive to manufacture than foam dielectric cable.
The assignee of the present invention, circa 1984, supplied to the Department of Energy, United States Government, for use in the Nevada atomic test range, a special purpose cable designed to have extreme water and gas blocking capability in order to prevent ingress and migration of radioactive contamination. The cable comprised a copper clad aluminum inner conductor and a corrugated aluminum shield surrounding a foam dielectric. To maximize water and gas blocking performance, the aluminum shield was annular corrugated and employed adhesive between the shield and the foam dielectric. The shield had a thick wall; for 0.5 inch OD cable, the wall thickness was 0.016 inch; for xe2x85x9e inch cable, the wall thickness was 0.020 inch or 0.025 inch depending upon the crush strength specified. The tungsten inert gas process used to weld the cable shield was almost an order of magnitude slower than the process capabilities of the cable of the present invention. For this reason, and a number of others, the cable was prohibitively costly and would not have been suitable for the mass consumption market.
Other aluminum annular helical corrugated cable is known, however, like the afore-described atomic test cable, it is characterized by having a thick-walled shield, for example, in the range of 0.016-0.020 inchxe2x80x94too thick to have the malleability needed in the practice of the present invention.
It is an object of the present invention to provide for the first time a cable which possesses all four of the above-stated desired attributes: 1) low cost; 2) electrical properties including shield effectiveness and intermodulation suppression comparable to that of solid tubular shielded cable; 3) mechanical properties, primarily flexibility comparable to corrugated cable; and 4) water blockage comparable to annular corrugated cable.
It is another object of the present invention to integrate in a novel and unique way an assemblage of cable material compositions, structural configurations and manufacturing processes to produce a coaxial cable with the lowest cost of any known cable with comparable electrical performance and flexibility.
It is another object of the present invention to produce such a cable having manufacturing cost comparable to that of braided shield cable products, and yet having the electrical properties, mechanical flexibility, and water blocking capability of more expensive coaxial cables.
It is an object to provide a helical corrugated coaxial cable possessing, for the first time, without the use of adhesives, water blocking performance exceeding any known helical corrugated cable not using adhesives or other special water blocking provisions.
It is still another object of the invention to provide a helical corrugated coaxial cable which can be manufactured at line speeds in excess of the line speeds of other known corrugated cable manufacturing processes.
It is yet another object of the invention to provide annular corrugated coaxial cable within which the formation of air gaps has been minimized or eliminated completely to thereby improve the water blocking performance of the cable compared to conventional annular corrugated cable.
It is yet another object of the invention to provide the first commercially practicable all-aluminum, foam dielectric, corrugated shield cable suitable in cost and performance for mass consumption.
While the present invention is susceptible of embodiments in various forms, there is shown in the drawings and will hereinafter be described some exemplary and non-limiting embodiments, with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiments illustrated.
In the disclosure, the use of the disjunctive is intended to include the conjunctive. The use the definite article or indefinite article is not intended to indicate cardinality. In particular, a reference to the xe2x80x9cthexe2x80x9d object or xe2x80x9caxe2x80x9d object is intended to denote also one of a possible plurality of such objects.